Beam scanners used for digital film recording are typically one of two types. A first type is a simple one-dimensional scanner, which is combined with means for moving film in a direction orthogonal to the scan direction, so as to record an image in a two-dimensional field on the film. Examples of this type of film recorder are given in U.S. Pat. No. 4,375,063 to Kitamura using a one-dimensional rotating polygon scanner and a rotating drum for moving the film, and U.S. Pat. No. 4,505,578 to Balasubramanian using an oscillating galvanometer mirror for one dimension of scanning and a braked gravity transport to move the film in the other direction at a uniform velocity.
The second type is a two-dimensional scanner having two scanning mirrors in series, each rotatable about an axis orthogonal with the other, so as to record an image in a two-dimensional field on stationary film. In A. C. Mecklenburg's article "Two-mirror, two-axis, rapid frame rate galvanometer scanning using a novel resonant scanner/dynamic focusing mechanism", SPIE, 1987, a scanning system is described which uses ROM lookup tables to correct distortions that would normally result from a two-mirror scanner. Referring to FIG. 4, the system includes a laser 10, a two-lens-combination beam expanding and focusing telescope 12 including a fast focus lens 14 and a slow focus lens 16, and two scanning mirrors, an X-mirror 18 that rapidly scans the laser beam across a Y-mirror 20, and the Y-mirror 20 which slowly sweeps the beam down the image plane 22. A pixel clock determines when spots are recorded.
Typically, two-mirror, two-dimensional scanners introduce a number of focus errors and distortions which must be compensated for. For example, if a spot in the center of the image is brought to a focus, the path of beam will be longer for spots away from the center and concentric rings of unfocus will result if the focus remains fixed. Mecklenburg includes a fast focus lens 14 moving in phase with the X scanning mirror 18 and a slow focus lens 16 stepping with the Y scanning mirror 20 to positions read from a ROM lookup table to keep the image spots in focus. Another error arises because the distance from the X scanning mirror 18 to the image plane changes as the beam is swept in a Y direction from top to bottom. Accordingly, when the angular amplitude of the X scanning mirror's motion is constant, the length of the line subtended by the image plane changes and lines at the top and bottom of the image are then longer than lines near the middle. This `pincushion` error is corrected by Mecklenburg with a ROM lookup table which determines the X scanning mirror's angular amplitude on a line-by-line basis. Another error arises because the beam intersects the image plane at a point whose position relative to the center is proportional to the tangents of the mirror angles. Accordingly, equal angular steps of the Y scanning mirror 20 and equal angular intervals of the X scanning mirror 18 for recording the image do not correspond to equal distances in the image plane. An additional complication in spacing pixels may also occur when the X scanning mirror 18 oscillates sinusoidally instead of being driven linearly. These tangential and sinusoidal pixel spacing errors are compensated for by Mecklenburg by using ROM lookup tables to determine the Y scanning mirror's position for each line and to modify the frequency of a voltage controlled oscillator that determines the time interval between pixels.
An object of the present invention is to provide a two-dimensional beam scanner which has high positional accuracy and scan spot uniformity without needing computer compensation, which is simple in construction and which is capable of high scan rates.